Lost my virginity to my Bestfriend who has a girlfriend

So two nights ago my Bestfriend and I were watching tv and we started making out. One thing led to another and he ended up taking my virginity. He has a girlfriend and feels so remorse but I feel so wrong for it. I’ve never met his girlfriend but I still feel guilty.

Edit: I came here for advice and everyone is calling me a hoe/slut/etc. Trust me: the past few days I’ve been calling myself all of those things too. I feel HORRIBLE but I’m in a tough situation because I don’t want to go behind my Bestfriend’s back and tell his girlfriend. I know I’m in the wrong, I just have no idea what to do. I’m scared he’ll be mad at me if I tell her and I’ll walk out of this situation without my virginity or my Bestfriend.